Inhibition of transmitted/founder (T/F) HIV-1 strains by MxB. (A) Ten T/F viruses were used to infect SupT1 cells that express MxB with doxycycline induction. The MxB protein has a C-terminal FLAG tag as described previously . Levels of newly produced HIV-1 were determined by infecting the TZM-bl indicator cells. Folds of inhibition by MxB were calculated by dividing the amounts of HIV-1 produced from MxB-expressing SupT1 cells with virus amounts produced by control SupT1 cells. Results shown are the average of three independent experiments. The T/F viral DNA clones were obtained from the NIH AIDS Reagent Program [16,17]. (B) PM1 cells were stably transduced to express MxB under doxycyline induction, and were infected with the above ten T/F HIV-1 strains. MxB inhibition was determined as described above. (C) Levels of exogenous MxB-FLAG in the above SupT1 and PM1 cell lines, as determined by Western blotting. Tubulin was probed as the internal control. (D) Infectivity of the T/F HIV-1. The T/F viruses were generated by transfecting the HEK293T cells. Viruses of the same levels of reverse transcriptase (RT) activity were used to infect the TZM-bl indicator cells. The averages of three independent infections are shown. (E) Alignment of HIV-1NL4–3 capsid sequence with those of the two T/F viruses CH040.c and RHPA.c. Red arrows indicate the three amino acid positions that are occupied by the same amino acids in CH040.c and RHPA.c but by different amino acids in NL4-3. (F) Effects of mutations H87Q, H120N and G208A on the infectivity of HIV-1NL4–3. Virus infectivity was determined by infecting the TZM-bl cells. Averages of three independent infections are shown. (G) Mutations H87Q and G208A render HIV-1NL4–3 resistant to MxB. The mutants were tested in MxB-expressing SupT1 cells for their sensitivity to MxB inhibition.